Add section on Time-dependent Brier Score to user guide

[sebp]

Copied from a previous blog post.

[codecov]

Codecov Report

Merging #220 (9773e22) into master (d2e5260) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files          37       37           
  Lines        3126     3126           
  Branches      458      458           
=======================================
  Hits         3074     3074           
  Misses         28       28           
  Partials       24       24           

Impacted Files	Coverage Δ
sksurv/metrics.py	100.00% <ø> (ø)

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